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Re: [Help-gsl] gsl_blas speedup

From: Brian Gough
Subject: Re: [Help-gsl] gsl_blas speedup
Date: Thu, 03 Jan 2008 11:57:59 +0000
User-agent: Wanderlust/2.14.0 (Africa) Emacs/22.1 Mule/5.0 (SAKAKI)

At Wed, 2 Jan 2008 15:44:40 +0100,
Riccardo Lucchese wrote:
> Anyway on calls like ddot this can be 2 times faster than now, and
> as in my tests simple functions like ddot don't take any advantage
> in using sse ecc..  (especially due to the calls overhead).
> Maybe I'm all wrong :) ? Any other ideas?

To do the equivalent of 'NOSIZECHECK' I would call the cblas_ routines
directly, instead of the gsl_blas routines.  This will eliminate the
size-checking overhead.

If function call overhead is still a problem (i.e. 'FORCEINLINE') then
your vectors must be quite small, otherwise the computation of the
BLAS routine itself will dominate.  

BLAS was really designed for large vectors. In the small vector case
some equivalent inline functions could help.  James Bergstra wrote
some sample inline BLAS routines a while back but I believe he decided
they did not give enough performance advantage unless the vector was
extremely small (like a few elements).  I think there could be some
cases (like DDOT as you say) where it could be worthwhile though.  It
would make sense to write such functions as an alternative to any
cblas_ library.

With small vectors one is usually working with a fixed rather than
variable length, which is another distinction from the usual BLAS

Brian Gough

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